Thermal comfort is driven by the temperature inside a person's body and the temperature at the surface of your body. Deep body temperature of a person is generally 37° C. and the mean skin temperature of a person is generally 33° C. People become uncomfortable in a thermal sense when the environment changes for example, increased wind or extra sunny day, or when a person moves to a cooler place. In common situations, discomfort in a thermal sense is felt by a user when the user's skin temperature changes, rather than a change in core body temperature.
Conventional chemical or electric heating systems used in clothing can easily deliver heat at relatively high levels. Most currently available devices consist of a wearable garment with heating pads that can be manually adjusted by the user. In at least some devices the heating pads produce an equal heat output and all the heating pads are activated to provide heat. Existing products are often bulky, heavy, require manual operation and are limited in their range of operation